Ignition coils are generally provided with a case in which an assembly of a primary coil, a secondary coil, and a center core and an outer peripheral core are arranged, and gaps in the case are filled with a resin. The shape of such an ignition coil depends on whether the assembly is arranged outside or inside of a plug hole. The configuration in which the assembly is arranged outside of the plug hole is advantageous in that the ignition performance can be designed without being affected by the constitution of the ignition coil, which depends on the inner diameter of the plug hole.
For example, PTL 1 discloses an ignition coil device for an internal combustion engine including a primary coil, a secondary coil, a center core and a side core which are buried in an insulation material and disposed on the outside of a plug hole. In this ignition coil device, the side core is covered with a core cover, with a stress relaxing material being arranged an outer peripheral part configuring the core cover. The stress relaxing material relaxes a stress generated in the insulation material due to the difference in thermal expansion between the core and the cover to prevent the insulation material from being cracked.